In recent years, various types of high-energy-density batteries have been actively developed as power sources for small portable devices, such as mobile phones; nighttime electric energy storage systems; household distributed electric energy storage systems based on photovoltaic power generation; electric energy storage systems for electric vehicles; and the like. In particular, since lithium ion batteries have an energy density exceeding 350 Wh/L, and are superior in safety and reliability, such as cycle characteristics, when compared to lithium secondary batteries using lithium metal as a negative electrode, the market for lithium ion batteries as power sources for small portable devices is dramatically growing. Lithium ion batteries use a lithium-containing transition metal oxide, such as LiCoO2, LiMn2O4, or the like, as a positive electrode active material, and a carbon-based material, such as graphite, as a negative electrode active material. At present, lithium ion batteries are being developed with increasingly higher capacities. However, increasing the capacity of lithium ion batteries by improving practically developed positive electrode oxides and negative electrode carbon-based materials has almost reached its limit. Thus, it is difficult to satisfy demands from device manufacturers for high energy density. Further, in a combination of a high-efficiency engine and an electric energy storage system (for example, in a hybrid electric vehicle), or in a combination of a fuel cell and an electric energy storage system (for example, in a fuel cell electric vehicle), in order for the engine or the fuel cell to be driven with maximum efficiency, it must be driven at a constant output. Thus, in order to handle load fluctuations and energy regeneration on the load side, high-power-discharge characteristics and high-rate charging characteristics are demanded on the electric energy storage system side. In response to these demands, in the area of electric energy storage systems, research and development is being performed to increase the output of lithium ion batteries, which are characterized by a high energy density, and on lithium ion capacitors to increase the energy density of electric double layer capacitors, which are characterized by high output.
On the other hand, regarding electric energy storage devices, such as lithium ion batteries and capacitors, techniques have attracted attention that increase the capacity and voltage of electric energy storage devices by doping lithium ions on active materials in advance (hereinafter referred to as predoping). For example, by applying predoping to a high-capacity material, such as an insoluble and infusible substrate with a polyacene-type skeletal structure as described in Non-Patent Literature 1, Patent Literature 1, Non-Patent Literature 2, Non-Patent Literature 3, and the like, it becomes possible to design an electric energy storage device that sufficiently utilizes its characteristic (high capacity), as described in Non-Patent Literature 4. This makes it possible to meet the demand for higher energy density and output in an electric energy storage device. Predoping is a technique that has long been in practical use. For example, Non-Patent Literature 5 and Patent Literature 2 each disclose an electric energy storage device having high voltage and high capacity, in which an insoluble and infusible substrate with a polyacene-type skeletal structure, which is a negative electrode active material, is predoped with lithium. For lithium-predoping, doping can be electrochemically performed by assembling an electrochemical system in which the electrode to be predoped is used as a working electrode and lithium metal is used as a counter electrode. However, in this method, it is necessary to take the predoped electrode out of the electrochemical system to be installed in a battery or capacitor. Therefore, as a practical predoping method, a method has long been used in which a lithium metal foil is laminated to an electrode containing an active material, thereby contacting them with each other, and after an electrolytic solution is filled, the active material is doped with lithium. This technique is effective for coin-type cells having a small number of relatively thick electrodes. However, in batteries having a structure in which multiple thin electrodes are laminated, or in batteries having a wound-type structure, production steps become complicated, and problems occur with the handling and the like of thin lithium metal. Therefore, a simple and practical predoping method is required.
As a method for solving the above problems, Patent Literature 3 to Patent Literature 6 disclose predoping methods that use a porous current collector (predoping method using porous current corector). For example, Patent Literature 3 discloses an organic electrolyte cell in which pores punching from the front surface to the back surface are provided, a negative electrode active material is capable of reversibly carrying lithium, lithium originating in the negative electrode is carried by electrochemical contact with lithium that is arranged to oppose the negative electrode or positive electrode, and the opposed area of the lithium is not more than 40% of the area of the negative electrode. In this battery, electrode layers are formed on current collectors provided with through-pores. By short-circuiting the lithium metal and a negative electrode arranged in the battery, lithium ions pass through the through-pores of the current collector after an electrolytic solution has been filled, thus doping all of the negative electrodes. An example of Patent Literature 3 discloses an organic electrolyte cell that uses an expanded metal as a current collector provided with through-pores, LiCoO2 as a positive electrode active material, and an insoluble and infusible substrate with a polyacene-type skeletal structure as a negative electrode active material. The negative electrode active material can be easily predoped with lithium ions from the lithium metal arranged in the battery.
Further, a method has been disclosed in which lithium metal powder is mixed in an electrode, or lithium metal powder is uniformly dispersed on a negative electrode as described in Patent Literature 7. After filling a solution therein, a local cell is formed on the electrode, thereby storing lithium uniformly in the electrode. Further, Patent Literature 8 discloses a method in which polymer-coated Li fine particles are mixed in a negative electrode to produce a negative electrode. After assembling a capacitor, the negative electrode is impregnated with an electrolytic solution, and the polymer portion of the polymer-coated Li fine particles is eluted in the electrolytic solution to cause conduction (short-circuiting) between the Li metal and the carbon of the negative electrode, whereby the carbon of the negative electrode is doped with Li.
Each of the above predoping techniques is a technique in which predoping is started in a cell by filling an electrolytic solution after a battery or a capacitor has been assembled. On the other hand, other methods are known, such as a technique in which an electrode is produced using an electrode material containing lithium, by immersing an electrode material in a solution in which n-butyllithium is dissolved in an organic solvent such as hexane, and by reacting the lithium with the electrode material (Patent Literature 9); a method in which lithium is reacted with graphite while the lithium is in a gas phase by an approach called a Tow-Bulb method, thereby causing graphite to contain lithium (Patent Literature 10); and a method in which lithium is mechanically alloyed through a mechanical alloying process (Patent Literature 10).